In general, a hydraulic system for a construction machine includes an engine generating power, a main hydraulic pump driven by receiving the power of the engine to discharge working oil, a plurality of actuators performing an operation, an operating unit operated so as to operate an actuator of a desired operating device, and a main control valve distributing working oil required by the operation of the operating unit to a corresponding actuator.
The operating unit forms a required value (flow rate) according to a displacement of an operation of an operator, and a flow rate of working oil discharged from the hydraulic pump is controlled by the required value. The operating unit includes, for example, a joystick and a pedal. As described above, the control of a flow rate of working oil is referred to as a flow rate control of the hydraulic system.
Further, in order to discharge working oil from the main hydraulic pump, rotation torque of the pump needs to be changed. The torque is referred to as pump torque. The pump torque T is calculated by multiplying a pump capacity by pressure P formed in working oil. The pump capacity is a flow rate of working oil discharged for one rotation of a shaft of the pump.
The capacity of the hydraulic pump may be varied by an inclination angle of a swash plate and revolutions per minute (rpm) of the engine. When an inclination angle of the swash plate is small, a capacity is small, and when an inclination angle of the swash plate is large, a capacity is large.
An inclination angle of the swash plate is controlled by a pump controller of a corresponding hydraulic pump. Further, when the rpm of the engine is large, a flow rate is increased, and when the rpm of the engine is small, a flow rate is decreased.
In order to rapidly operate the actuator in a state where a working load is not applied to the actuator, the hydraulic pump is controlled by the pump controller so that a flow rate is increased. By contrast, in a state where a large working load is applied to the actuator, in order to meet limited torque of the engine, the hydraulic pump is controlled by the pump controller so that a flow rate is decreased. The control of the pump torque implemented by the hydraulic pump is referred to as horsepower control of the hydraulic system.
In the meantime, the actuator includes a linear actuator, in which a rod linearly moves and a hydraulic motor, in which a shaft rotates.
In the linear actuator, a piston rod is inserted into a cylinder, and first and second ports are formed at both sides of the cylinder. When working oil is supplied to the first port at one side, the piston rod is pushed by the working oil, and the working oil is discharged through the second port by the pushed piston rod. However, a flow rate of the working oil entering through the first port is different from a flow rate of the working oil discharged from the second port. The reason of the difference in the working oil is a difference by a cross-section area of the piston rod. More specifically, the cylinder having no piston rod has a large cross-sectional area corresponding to an internal diameter of the cylinder, and the cylinder having a cylinder rod has a small cross-sectional area corresponding to a cross-sectional area obtained by subtracting a cross-sectional area of the cylinder rod from the internal diameter of the cylinder, so that the flow rates of the working oil at both sides of the piston rod are different due to the difference in the cross-sectional area.
As described above, there is a difference between the flow rate of the inflow working oil and the flow rate of the discharged working oil when the actuator is driven, so that there is a problem in that an operation speed of the actuator is decreased due to the difference in the flow rate of the working oil.
More specifically, a charging hydraulic circuit is configured to supplement a flow rate from a side, at which the flow rate is excessive, to a side, at which the flow rate is insufficient, and an operation speed of the actuator is decreased during a process of charging the working oil.